The long term evolution (LTE) project is the greatest new technology research and development project initiated by the 3 GPP in the recent two years, which improves and enhances the air access technology of the 3G. Compared with the 3G, the LTE has more technical advantage, which manifested in the aspects such as a much higher user data rate, grouping transmission, system delay decrease, an improvement of system capacity and coverage, and reducing of the operating cost, etc.
The LTE downlink uses the orthogonal frequency division multiplexing (OFDM) technology. The OFDM has the characteristics such as high usage of frequency spectrum, anti-multipath interference, etc. and the OFDM system can effectively resist the influence produced by the wireless channel. The LTE uplink transmission scheme uses a single carrier frequency division multiple access (SC-FDMA) with a cyclic prefix. In the transmission scheme in which the uplink uses the SC-FDMA with the cyclic prefix, a frequency domain signal is obtained by using a discrete fourier-transform (DFT), then a frequency spectrum shifting is performed after inserting a zero symbol, and an inverse fast fourier transform (IFFT) is performed on the shifted symbol, which can reduce the peak-to-average power ratio of the transmitting terminal. Therefore, the SC-FDMA system is also called as a discrete fourier-transform spread orthogonal frequency division multiplexing (DFT-S-OFDM) system.
For the multi-carrier system, the offset of the carrier frequency could cause generating the interference between the sub-channels. A plurality of orthogonal sub-carriers exist in the OFDM system, and the output signal is the overlap of a plurality of the sub-carrier signals. Due to that the sub-carriers overlap each other, this requires a higher demand for the othogonality of the carriers.
Due to the moving of a terminal, a Doppler frequency shift could be generated between a base station and the terminal. In the mobile communication system, especially in the high speed scenario, this kind of frequency shift is particularly obvious. The Doppler frequency shift will generate a frequency error between a receiver and a transmitter, which causes an offset occurring in the received signal in the frequency domain, introduces the interference between the carriers, thus increasing the error rate of system and worsening the performance.
The size of the Doppler frequency shift is related to the velocity of the relative motion, and the relationship between them is:
      f    d    =            -                        f          0                C              ×    v    ×    cos    ⁢                  ⁢    θ  
Wherein, θ is a separation angle between the terminal moving direction and the signal transmission direction; ν is the terminal moving velocity; C is the electromagnetic wave propagation transmission velocity; and f0 is the carrier frequency.
For the mobile terminal, the LTE system ensures to keep the system feature of the mobile user with a velocity of 15 km/h and below to be best, while provides a high performance service to the mobile user with a velocity of 15˜120 km/h, keeps the service to the mobile user with a velocity of 120˜350 km/h, and keeps the mobile user with a velocity above 350 km/h to be connected. In the scope of this velocity, the Doppler frequency shift is above 400 Hz. The base station and the terminal must support enough frequency offset compensation technology to satisfy the service quality requirement.
For the receiver, the function which the receiver must finish is to estimate the frequency error between the receiver and the transmitter and finish the frequency error correction. The frequency shift for the terminal receiving is fd and the uplink signals are sent after the terminal locks the downlink signal frequency, and the frequency shift for the uplink receiving is 2*fd.
As shown in FIG. 1, the relative motion directions of the terminal and the base station are different, which could generate positive and negative frequency offsets. Provided that f0 is the transmission frequency of the base station, when the terminal moves to the direction away from the base station, it will generate a negative frequency offset −fd, the frequency for the terminal to receive is f0−fd, while the frequency for the base station to receive is f0−2*fd; When the terminal moves to the direction towards to the base station, it will generate a positive frequency offset fd, the frequency for the terminal to receive is f0+fd, while the frequency for the base station to receive is f0+2*fd. When the terminal moves between the two base stations from one base station to another base station, a frequency hopping, which is from the frequency f0−fd to the frequency f0+fd, could occur in the terminal, and the terminal will have a frequency hopping 2*fd. The 2*fd is not a small challenge not only for the base station receiver but also for the terminal receiver. The overlarge frequency offset could cause a communication quality reduction, and could cause the service interruption in severe cases, especially in the high speed mobile environment.
If the frequency offset cannot be estimated correctly and be corrected, then the system performance will reduce greatly. Especially it will be more obvious when the frequency offset is larger (the corresponding moving velocity of the terminal is higher). Therefore, it is very important to find a method and an apparatus for estimating and compensating the frequency offset with a good performance of correcting the frequency offset and a stable implementation for the engineering implementation.